This invention relates to a method and apparatus for electrically testing cables comprising a plurality of insulated conductors in an automated fashion. The effective electrical isolation between individual conductors in a cable as well as between individual conductors and ground are measured. The invention also provides for cable failure mode analysis during flame testing of a multi-conductor cable.
FIG. 1 illustrates a sampling of the various types of cables comprising multiple insulated conductors. Although the conductor cores are illustrated as solid, often each conductor core future comprises individual small diameter conductors in physical contact with the other conductors of a core. Such conductor cores are identified as “stranded” in the art. Stranded conductor cores typically contain 7 or more individual conductors or strands. The method and apparatus of the present invention is suitable for cables having insulated solid conductor cores as well as cables having insulated stranded conductor cores.
Insulation resistance is used as the preferred measure of electrical isolation. As a unique signature voltage is impressed on each conductor in a cable (or cable bundle) then by systematically allowing for and monitoring known current leakage paths it is possible to determine if leakage from one conductor to another, or to ground, is occurring. Part or the entire voltage signature may be detected on any of the other conductors in the cable (or in an adjacent cable) or may leak to ground directly.
Application of this principle is illustrated with a three-conductor (3/C) cable in FIG. 2. If 100 volts is applied to conductor 1, then the degree of isolation of conductors 2 and 3 from conductor 1 can be determined by systematically opening a potential conductor-to-conductor current leakage path and then reading the voltages of each conductor in turn while conductor 1 is energized. Determining the insulation resistance between conductors 1 and 2 at the time of voltage measurement on conductor 2 is a simple calculation employing Ohm's law:                               I                      1            -            2                          =                              V            2                    R                                    (        1        )            And                               R                      1            -            2                          =                                            V              1                                      I                              1                -                2                                              -          R                                    (        2        )            
In FIG. 2A, three conductors 1, 2 and 3 are illustrated. A known supply voltage V1 is being applied to conductor 1. Leakage current I1-2 is detected by measuring the voltage V2 across known resistor R. Insulation resistance R1-2 is a measure of the electrical isolation between conductor 1 and 2. Similarly, insulation resistance R1-3 is between conductors 1 and 3. Insulation resistance R2-3 is between conductors 2 and 3.
In the same way, the insulation resistance existing between conductors 1 and 3 at the time V2 is measured can be determined. A time-dependent history of R1-2 and R1-3 can be obtained by continuously switching between the two conductors and recording the voltage drop across R at each switch position. (Of course an alternate method would be to connect a resistor/voltmeter assembly to both conductors 2 and 3 simultaneously and keep a continuous record of the two voltages. This approach quickly becomes unwieldy as the number of conductors increases.)
This method of measuring insulation resistance R1-2 is not strictly correct as it ignores the alternate leakage path from conductor 1 to conductor 3 and then from conductor 3 to conductor 2. When the additional leakage path is considered, the above equations change to:             I              1        -        2              +          I              1        -        3        -        2              =            V      2        R  and       R          1      -      2        =                    V        1            -              R        *                  (                                    I                              1                -                2                                      +                          I                              1                -                3                -                2                                              )                            I              1        -        2            
With two equations and three unknown quantities (R1-2, I1-2, I1-2-3) it is not possible to directly determine insulation resistance R1-2. What must be added are additional measurements, with switch SW in the alternate position, as illustrated in FIG. 2B.
However, measuring voltages with switch SW in both positions does not describe the isolation existing between conductors 2 and 3 (because conductor 1 is always the energized conductor). However, by sequentially energizing each conductor and reading the impressed voltages on the remaining conductors one can determine the relative resistance existing between any conductor pair. An example circuit (see FIG. 3).
FIG. 3 schematic illustrates how a three conductor cable's insulation resistances are measured using two sets of controlled switches, one set on the input side (SWi) and one on the output side (SWj) of the circuit. One switch on the voltage input side is closed (thereby energizing one conductor) followed by the sequential closing-measurement-opening of each measurement side switch. Each sequential switching configuration measures leakage currents between one energized “source” conductor and one non-energized “target” conductor, and the various pairs are systematically evaluated in sequence.
The addition of a ground plane further complicates insulation resistance measurements. FIG. 4 illustrates additional insulation resistance R1-G, R2-G and R3-G between each respective conductor and the ground plane. The schematic of FIG. 3 is not able to determine the resistances R1-G, R2-G and R3-G. It is seen then that a measuring method and apparatus are needed to conveniently measure all of the insulation resistance's in a multi-conductor cable in the presence of a ground plane
Another aspect of the present invention is providing a convenient means of testing cable failures during fire exposure and to qualify cables for use in high-reliability applications. For example, The U.S. Nuclear Regulatory Commission (USNRC) continues to focus considerable attention on the issue of fire-induced cable failures and the associated circuit faults induced by such failure. Of particular interest is the potential that fire-induced failure of a control cable might lead to the spurious operation of plant equipment. This issue is of interest both in the regulatory process and with regard to fire probabilistic risk assessment (PRA) methods and data.
In the context of quantifying the impact of cable failure modes and effects on fire risk analysis, one quantity that needs to be estimated is the likelihood that spurious equipment operations may occur during a fire. The likelihood that a given circuit fault mode (e.g., spurious operation) will be observed is tied intimately to the likelihood of various cable failure modes (e.g., hot short, short to ground, open circuit) will occur as a result of fire-induced cable damage.
For example, given a control or power circuit with a grounded power source, spurious operation circuit faults would typically be observed only during a conductor-to-conductor hot short. Furthermore, the hot short would typically need to involve specific combinations of conductors. The specific conductor shorting configurations that might lead to spurious operation represent some subset of all of the possible conductor shorting combinations.
Other conductor shorting combinations or other modes of shorting might lead to other circuit fault modes. For example, shorting of an energized conductor to ground might cause the protective fuses to open, thus de-energizing the circuit. Hence, in order to systematically estimate the likelihood of a spurious operation circuit fault given cable failure it is necessary to (1) identify the various conductor failure modes and their associated effects on the circuit, and (2) assess the relative likelihood that the cable failure modes that lead to spurious operation circuit faults might actually occur.
Measuring the insulation resistance values illustrated in FIG. 4 satisfies the need to systematically estimate circuit faults provided the measurements can be made during a failure event. A cable to be tested is connected to test terminals inside an environmental flame test chamber. Insulation resistance measurements are taken repeatedly until cable faults are identified. The time to fault as well as the type of fault is then used to qualify a cable for use in high-reliability service.
Simulation of real-world failures is closest represented by using circuit energization equal to voltages encountered in actual use circuits. For example 4-20 ma control circuits are typically energized with a 24 V DC power source. When placed in cable tray, they may be adjacent to motor control cables operating at 120 V AC. For this case, real-world simulation will use the higher 120 V level to estimate all circuit failures, or the lower 24 V power source to estimate circuit failures is, isolated 4-20 ma cable wiring.
In either scenario, an enormous amount of labor must be expended in taking and recording the numerous readings during the course of cable fire exposure. For safety, personnel must also be isolated from the hazards of the fire exposure test, including heat, smoke and toxic vapors. The three conductor cable of FIG. 4 has six insulation resistances to be determined throughout a flame test. Table 1 shows that as the number of conductors increases, the number of measurements increases geometrically. In the case of a ten conductor cable, 55 insulation resistances need to be determined. This increases to 136 for a sixteen conductor cable, 300 for a 24 conductor cable and 528 for a 32 conductor cable.
TABLE 1Number of Insulation Resistances in Multi-conductor CableNo. of InsulationNo. of InsulationTotal Number ofResistances,Resistances,InsulationNumber ofConductor toConductor toResistanceConductorsGroundConductorMeasurements 2 2 1 3 3 3 3 6 4 4 610 5 51015 6 61521 7 72128 8 82836 9 9364510104555121266781616120 136 2020190 210 2424276 300 2828378 406 3232496 528 
During environmental chamber testing, personnel hazards of heat (thermal burns) and toxic fumes and gases are generated. Toxic fumes and gases originate with the flame as will as cable insulation combustion and partial combustion products. Steps taken to minimize personnel exposure are limited when measurements are being directly identified and recorded by personnel. In order to allow total separation of the environmental testing from personnel a fully automatic resistance measuring apparatus is required.
As a result it, there is a need in the art for an apparatus and method that measures multi-conductor cable insulation resistances automatically while protecting personnel from hazards of environmental testing and is useful with multi-conductor cables comprising more than 3 conductors.